Shavua Tov. Regarding today’s Parsha, there are two consecutive verses that don’t seem to make sense.
The first Pasuk says that Shabbat needs to be observed because it is a holy day. This is followed by, מחלליה מות יומת, and those who desecrate it, shall surely die. The death penalty alluded to is death by stoning.
The very next Pasuk says that anyone who does מלאכה on Shabbat, ונכרתה הנפש ההיא מישראל, that soul should be cut off from his people. This is the punishment known as כרת.
Doing a מלאכה, loosely translated as work, is the clearest violation of Shabbat.
So we have one verse that says סקילה is the punishment and the other says כרת is the punishment for violating Shabbat. The explanation is that if one was properly warned and there were two witnesses, he would get סקילה. If he was not warned and there were no witnesses, but he knew of his transgression, he would get כרת.
If one violated Shabbat in error as he did not know something was forbidden, or he forgot it was Shabbat, he would owe a קרבן חטאת, a sin offering.